The present invention relates to gas sensors. More particularly, the present invention relates to an externally crimped gas sensor and a method for forming an external shield crimp.
Exhaust gas sensors (or exhaust constituent sensors) have been used for many years in automotive vehicles to sense the presence of constituents in exhaust gasses (namely oxygen) and to sense, for example, when an exhaust gas content switches from rich to lean or lean to rich. One known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner.
Oxygen sensors are used in the automotive industry to sense amounts of oxygen present in exhaust gases relative to a reference gas, such as air. A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode which is exposed to the exhaust gas, and a reference electrode which is exposed to the reference gas. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          xe2x80x83        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromative force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control about stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. In addition to their value for emissions control, the sensors also provide improved fuel economy and drivability.
Because automotive exhaust sensors are mounted to members of the vehicle exhaust flow system, the sensors must be durable, be able to withstand vibration and jarring such as would occur during installation and normal vehicle operation, and be able to withstand shock from the occasional stone or other small road debris that may happen to be thrown at the sensor, for example, by the vehicle""s tires.
Typically, great care is required when packaging and holding the flat plate sensing element within the outer housing (body) of the exhaust sensor. The flat plate sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. Accordingly, efficient assembly of exhaust sensors such that they are durable and leak free constitutes a substantial concern in the industry.
Protection from contamination and water intrusion into the sensor through all metal/metal interfaces is critical to have long term durability. One such interface is attachment of the external sensor shield, which protects the portion of the exhaust sensor protruding from an exhaust line, to the mounting, or shell, which is threaded to seat the exhaust sensor into a bushing formed in the exhaust line.
Conventional sensors use one of two methods for creating a metal/metal interface between the external sensor shield and the shell. One such method includes welding. However, welding procedures involve significantly high cost processes and complex equipment. Further, production requires long cycle times that are not amenable to low capital investment, lean modules.
A second conventional method involves crimping of the shield inside the shell. The seal provided by this crimp is aided by the reflexive action of the more flexible crimped shield against the inner surface of the more rigid shell. However, labor costs are increased due to process limitations in forming the crimp. Specifically, the process requires that the final exhaust sensor assembly, including the wiring harness be assembled completely on the lower subassembly and that the external sensor shield be crimped after other assembly is completed.
Accordingly, there remains a need in the art for a durable gas sensor having improved resistance to external contaminants and moisture that can be easily manufactured at a low product cost.
The problems and disadvantages of the prior art are overcome and alleviated by the method for making an exhaust gas sensor. The method comprises positioning a least an end portion of an upper shield having a wiring harness assembly that comprises an upper portion of the sensing element, over at least a first end of a shell whereby the wiring harness assembly and the subassembly converge; and crimping the upper shield end portion over the shell first end.
Further, the gas sensor comprises a sensing element, having an upper portion disposed in electrical communication with a wiring harness assembly and a lower portion disposed within a subassembly; an upper shield disposed around the wiring harness assembly; and a shell disposed around the subassembly, wherein a first end of the shell is concentrically disposed within and crimped to an end portion of the upper shield.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.